Publication DE 10 2010 015 498 A1 discloses a hydrostatic transmission, in which a left-hand and a right-hand drive motor arrangement are supplied via a common adjustable pump in a common closed circuit. The two hydraulic motors on each side and each drive motor arrangement are connected to one another in series. The two upstream hydraulic motors are driven in the course of the normal operation of the vehicle concerned in two-wheel drive mode, whereas the two downstream hydraulic motors are essentially only idling. A quantity of pressure medium is discharged constantly from the connecting lines between the two hydraulic motors on each side for this purpose, so that the respective upstream hydraulic motor converts no energy or only little energy and delivers no torque or only little torque. A common or a respective constant flow regulator, which is referred to below as a flow control valve, is connected to the connecting lines for this purpose.
Automatic switching from two-wheel drive mode to four-wheel drive mode takes place if the slippage of an upstream hydraulic motor increases, and if the corresponding wheel accordingly “spins”. The downstream hydraulic motor is then also supplied with pressure medium in such a way that this also converts energy and delivers torque in the sense of an all-wheel drive system.
Publication DE 10 2011 118 255 A1 discloses a similar hydrostatic transmission, of which the performance is improved by a control valve in the event of steering movements by the vehicle concerned. Said valve permits the equalization of the pressure medium between the connecting lines of the two drive motor arrangements, that is to say from the connecting line of the curve inner wheel to the connecting line of the curve outer wheel.
Disclosed in DE 10 2013 211 621 A1 is a drive motor arrangement for one side of a hydrostatic transmission having an upstream hydraulic motor and a downstream hydraulic motor in a closed circuit. The publication discloses an integration of the flow control valve connected to the connecting line into the housing of one of the hydraulic motors.
A disadvantage associated with suchlike drive motor arrangements is that their hydraulic motors always have a fixed displacement, so that a constant speed of travel, that is to say only a single transmission step, results at a constant supply of pressure medium.